The present invention relates to paper, pulp, board or other similar fibrous web machines. More specifically, the present invention relates to calenders of fibrous web machines and, in particular, the present invention relates to a method for calendering a fibrous web in a calender which includes at least two roll stacks which each have at least three rolls, and in which calender the fibrous web is passed to run between each roll pair of each roll stack. The invention also relates to a calender which includes at least two roll stacks which each have at least three rolls, and in which calender a fibrous web has been passed to run between each roll pair of each roll stack.
Calendering is a method by means of which attempts are generally made to improve the properties of a web-like material, such as a paper web, in particular, its thickness profile, smoothness, gloss, surface porosity and transparence. In the calendering process, a paper web is passed into a nip formed between rolls pressed against each other, in which nip the paper web is deformed due to the effect of temperature, moisture and nip pressure, whereby the physical properties of the paper web can be affected by regulating the above-mentioned parameters and the time of action. The good physical properties achieved by calendering result in improved print quality, thus giving a competitive advantage to the paper manufacturer.
Up to the mid 1990s, calenders comprised three main categories, which are typically single- or double-nip hard-nip calenders and soft calenders as well as multi-nip supercalenders. All these calender types have their advantages and disadvantages.
Shoe or so-called long-nip calendering has been found to be generally good for producing low-gloss paper grades, i.e. grades having a Hunter gloss % below 40. When a higher gloss is required, the nip pressure is, however, not sufficient to produce gloss. With continuously increasing running speeds, calendering is becoming a bottleneck in the papermaking process and adequate quality is not achieved by means of today's hard-nip calenders. The drawbacks of today's papermaking process also include the fact that the loss of bulk increases when gloss is improved and that to achieve adequate quality it is necessary to employ abundantly coated webs and/or use off-line calendering, in particular soft calendering and/or multi-nip calendering, whose known application is supercalendering. The direction in the processing of the fibrous web and thus also in calendering is, however, towards on-line arrangements to an ever-increasing degree, also when the intention is to produce higher-quality printing paper grades, such as SC and glossy coated papers. FIG. 2 illustrates with a broken line currently available different paper grades, which include, among other things,                NP (NewsPrint) newsprint paper grades,        SC (SuperCalendered) paper grades,        MFC (Machine Finished Coated) fine paper grades,        LWC (Light Weight Coated) paper grades, and        WFC (WoodFree Coated) fine paper grades,and today's calenders, which include        a hard-nip i.e. machine calender,        a soft calender, and        a supercalenderfor producing different paper grades. It can also be seen from FIG. 2 that a multi-nip supercalender is remaining as the only alternative when the aim is to manufacture high-quality, i.e. high-gloss and smooth, WFC, LWC and SC paper grades.        
The good characteristics of the calendering technique common today are beginning to reach their physical limits and the surface properties of the fibrous web cannot be much improved any more without a risk of the surface being scratched. When running speeds simultaneously increase to be as high as 1600 m/min or more, a drawback is the shortening time of action of calendering, which leads to considerable capacity problems when producing high-quality paper grades, so that even three supercalenders are not necessarily sufficient to produce high quality at high speeds, but even a fourth supercalender is needed, which is expensive both as an investment and in respect of operation.
Because of the development of the soft calender technology, polymer-coated rolls can be used in the calender today. A problem associated with this is, however, that if more than three rolls in the supercalender are provided with an elastic polymer coating, the quality of the fibrous web begins to deteriorate. To meet increasing quality requirements, three new calender types have been developed, which are the OptiLoad® calender developed by Metso Paper, Inc., provided with a single roll stack and typically including 6-12 rolls and having as a special feature the possibility of affecting the linear loads of individual nips and providing the roll stack with a nip load even inversely increasing with respect to the force of gravity; the Janus® calender developed by Voith-Sulzer, provided with two roll stacks and typically including 6-10 rolls; and the Prosoft® calender developed by Küsters-Beloit, provided two roll stacks and typically including 6-14 rolls.
These new type of multi-nip calenders, which can be used both as on-line and off-line calenders, are standard types in today's calendering technique and they are based on enhanced utilization of an elastic roll coating, but they differ substantially from one another in respect of attainable calendering results.